As I've said before in other articles in this site,
the 351M/400 is one of the most over-looked and under-rated engines Ford
ever built. However, with the proper components it can be made to be a
real screamer. I researched the high performance buildup of my engine for
months and I discovered how to make one of these engines capable of
blowing the doors off of just about anything else out there, all for less
than the price of a "crate motor".

There are four basic factors with the 351M/400 that need to be
addressed to accomplish a true high performance buildup. They
are:

By far, the best thing you can do to your engine is to raise the
compression ratio. All of the 351M/400s came with 8.0 to 8.4 to 1
compression ratios (except for the 1971 400, which had a 9.1:1 CR). While
even a 9.1:1 compression ratio pales in comparison to the muscle car area
of the '50s, '60s & early '70s, that's about all you can feasibly
squeeze out of a 351M/400 motor on the 92 or 93 Octane gasoline available
today. If you go much higher than say 9.5:1, then you will have problems
with pre-detonation (also called "pinging") w/o the use of octane
boosters. Raising an engine's CR will give it the following
benefits:

More Power -- Raising an engine's CR from, say 8.5 to
9.5 to 1 (or 1 full point) will compress the air/fuel intake charge
by 11.8%. The formula for this
is:

Where NCR= New Compression Ratio and OCR= Old Compression
Ratio.

For example:

Compressing the intake charge by 11.8% "squeezes" the same amount
of air and fuel into a smaller volume. The more dense the air/fuel
charge, the more energy will be released when the charge is ignited by
the spark plug.

Better Gas Mileage -- Raising the CR effectively
increases the engines volumetric efficiency (VE) by producing more
energy from the same amount of air and fuel taken in by the engine
thereby increasing the engine's HP. In theory, if the engine is more
efficient and produces more HP, then the engine will have to do less
work at any given RPM to do the same job as an engine with a lower
CR.

So what's the best way to raise your engine's compression? Well,
there isn't an easy answer to this question and it also depends on how
much money you have to spend. Having said that, there are three ways to
increase your engine's CR. They are:

Replace your original cylinder heads with higher compression
heads that already have a smaller CC volume.

Install a set of high CR pistons with a smaller dished
center.

Shaving (milling or decking) the heads

Cylinder heads tend to warp slightly over time. A quality cylinder
head rebuild will include a procedure where a small amount of material
(around 0.010") is milled, or shaved, off of the bottom of the heads so
that they will lay perfectly flat on the engine block. Shaving 0.010" will
increase the compression ratio by approximately 2.3%. So, if your stock
400M has 8.5 to 1 CR, it will now have approximately 8.7 to 1 CR. About
the most material you can reasonably remove from your stock 351M/400 heads
is 0.025", which would increase your CR by approximately 5.75% which would
give a 400 a CR of 8.98875.

Installing a different set of heads

Since the 351M/400 engines are in the 335 engine series, every
single cylinder head from any engine in the 355 family will all
interchange with these engines. That leaves at least 6 distinctly
different types of cylinder heads that will work on the 78 & 79
351M/400. Below is a chart that list some of the 335 series heads along
with each head's CC (combustion chamber volume) that can be used on the
351M/400.

As you can see from the chart above, the best head you
can install on the 351M/400 is the Australian closed-chamber 302 2V head.
Just by bolting on this head, a 400 will develop 10.75 CR and a 351M will
develop 10.12 CR. However, these heads are EXTREMELY rare and you
will have to pay a hefty price if you can find a pair.

The 70-74 351C 2V heads will give your engine a slight
increase in compression, however, the small increase in CR is not worth
swapping the heads, in my opinion. You can get nearly the same increase in
compression by milling your stock heads the standard 0.010" when you have
them rebuilt. However, if you were to buy set of 351C heads and have them
milled by 0.025", then you can raise the CR on a your engine by nearly 3/4
of one full point!

The 71-74 351C 4V heads, like the 2V heads, will not
give your engine enough increase in CR to warrant changing your heads.
However, the 70-71 CJ (Cobra Jet) 4V heads will give your engine a
considerable increase in CR. Unfortunately, the 351C 4V heads are not a
good choice for a low RPM truck engine. The 4V heads have much larger
intake and exhaust ports (2.19" intake & 1.71" exhaust vs. 2.04"
intake & 1.66" exhaust) than the 2V heads. These heads were
designed for VERY HIGH RPM and do not produce good low-end torque. The
Bronco is much too heavy to make use of these heads.

Installing a set of high CR Pistons

Another way to increase your CR is to install a set of
high CR pistons. Unfortunately, there are not may choices available for
the 351M/400 engines. One option is to have a custom set of pistons made
for your engine. There are many companies, such as J & E and Keith
Black, that will make a custom set for your engine in any CR you want, but
be prepared to pay a minimum of $700.00 for a set of eight and to wait for
4 to 6 weeks for them to be made. However, there are a couple of other
much less expensive options available for both the 351M and the
400.

351M -- The only production high CR
piston for the 351M is the TRW 8.6 to 1 CR piston (P/N L2466F). This is a flat top forged
aluminum piston that is available in both standard and 0.030" sizes.
This piston will give a 351M with stock heads a 8.6 to 1 CR in the
standard size and a 8.7 to 1 CR in the 0.030 size.

400 -- The 1971 400 had a 9.1 to 1 CR
from the factory. This engine used a flat-top piston that was not
dished like the 72-82 400s. There are a couple of companies that I
know of that manufacture this piston. Badger Pistons by
Dyna Gear (http://www.dynagear.com/) manufactures a 9:1
flat-top cast aluminum piston for the 400. This piston retails for
around $123.00for a set of 8. Also, one of my readers
informed me that Ohio Piston & Pin manufactures a 9:1
flat-top piston (P/N 1282P) for
the 400 that retails for $138.00. The engine shop that built my
engine used the Badger Dyna Gear pistons. When you combine these
pistons with milling the heads, you can easily get 9.5 to 1 or more
compression ratio out of your
400.

The next best thing you can do for your engine is to
install a better camshaft and valve train. The 78 & 79 351M/400 used a
retarded camshaft and timing chain to meet stringent pollution standards
(see Dispelling the Myths & Untruths surrounding the
351M/400) from the factory. The stock cam and timing chain are, for
lack of better words, pure unadulterated garbage. Fortunately, there are
many aftermarket companies that offer high-performance cams & timing
chains designed for the 351C and the 351M/400. I recommend using one of
the newer dual-profile or roller camshafts available from companies such
as Crane Cams and Comp Cams. They are very expensive, but you would
not believe the benefits you will receive from your engine with one of the
newer types of cams available today. However, ANY quality
aftermarket camshaft kit will be a SIGNIFICANT improvement over the
stock cam and timing chain.

I used the Comp Cams 255 DEH Dual Energy cam kit on my
engine. This cam has a useable RPM range of 1000 to 5500 RPM, gets good
gas mileage, and makes a great RV or towing cam. I also recommend buying
the K-kit (P/N K32-206-3) which includes
lifters, matched valve springs, valve spring retainers, valve locks, valve
stems seals, dual-roller timing chain set, assembly lube, decals, and
instructions. No matter which manufacture you choose to use, make sure you
buy the matched valve train kit with your cam. Don't try to mix and match
parts!

Another good choice is the cam that was used on a 79
400 in an article entitled "Down & Dirty" that appeared in the
September 1998 issue of Hot Rod Magazine. They used the Comp
Cams X-Treme Energy #268 camshaft. This cam is a little more aggressive
than the one I chose with a useable RPM range of 2,000 to 5,200. Hot Rod
claims that this engine put out an impressive 382 HP and 456 ft-lbs of
torque! As soon as I have my engine run on a dyno, I'll post the results
in this article.

One other item you can upgrade in your valve train are
the rocker arms. I used the Comp Cams 1.72 ratio Magnum Roller rocker arms
in my engine (P/N 1411-16). You will
have to convert your engine over to hardened pushrods, screw-in studs and
guide plates to use these rocker arms. They are the perfect complement to
a dual-profile or roller cam shaft. Roller rocker arms are more accurate,
more efficient, and produce less friction than the stock rocker arms. Comp
cams claims that these rocker arms can give your engine 15 to 30
additional horse power.

Installing dual exhaust and a high performance
camshaft in your engine and leaving the stock 2V carburetor in place is
like inhaling through your nose and exhaling out of your mouth. Just try
it! It doesn't work very well, does it? Well, it doesn't work very well in
your engine either. Fortunately, there are many aftermarket 4V intakes and
4V carburetors for the 351M/400 engines. You can even install an
aftermarket fuel injection system like the ones available from Holley,
however, they are very expensive and I'm not convinced that they are much
better than carburetors.

Choosing an intake manifold is easy. I recommend a
dual plane manifold such as the Edelbrock Performer 400 EGR (P/N 3771) or the Weiand Action +Plus (P/N 8010) or the new Weiand Action +Plus EGR
(P/N 8013). The Edelbrock intake has a
good carburetor mounting surface area and will help keep your Bronco
emissions legal if you keep the EGR Valve. Both Weiand manifolds also have
a good carburetor mounting surface area but the 8010 does not have a
provision for an EGR valve. If you are not concerned with keeping your
Bronco emissions legal, then go with the Weiand 8010 manifold.

Choosing the correct carburetor for your engine is a
little more difficult. The key to proper carburetion is to choose a
carburetor that correctly fits your application. There are many different
carbs that will work on a 351M/400 engine. Listed below is a chart with
the most common carbs available, the CFM (cubic feet / minute), the type
of choke, the listed application, and the average retail price.

Manufacture

Carb model/ P/N

CFM

Choke

Application

Retail Price

Edelbrock

1406

600

electric

emission w/EGR

$250.00

Edelbrock

1411

750

electric

non-emissions

$270.00

Carter

9637

625

electric

Universal Ford w/AT kick down

$245.00

Holley

80457S

600

electric

Universal Ford w/AT kick down

$220.00

Holley

1850S

600

manual

Universal Ford w/AT kick down

$203.00

Holley

80508S

750

electric

Universal Ford w/AT kick down

$270.00

Holley

80452

600

electric

77-80 351M/400 Emissions legal

$250.00

Holley

80453

600

electric

77-78 351M/400 Emissions legal

$236.00

The formula below can help you determine the best size of
carburetor to use with your engine.

I think the mistake people make the most when
installing a carburetor is choosing a 750 CFM carburetor on an engine that
clearly doesn't need that much carburetion. In the example above, even a
fully modified 400 producing 100% VE would only need a 636 CFM
carburetor at 5,500 RPM. Be that as it may, many engine builders recommend
using a 750 CFM carburetor on a high-performance 400 CID engine. So, a 750
CFM carb should work fine on hi-output 400, however, on a 351M and on
stock-to-mild 400s, a 600 to 650 CFM carb should meet the maximum CFM
requirements of your engine.

I chose to use a Holley model 80457S on my
high-performance 400. It has fair throttle response, gets better gas
mileage than the factory 2V carb, and doesn't have any hesitation in it at
all. Because my engine has been modified, I did have to make a few tuning
changes to the carb to get it to work properly on my engine. The 80457S
comes with #65 main jets and a #25 accelerator pump nozzle. Holley
recommended using #66 or #67 main jets and a #28 accelerator pump nozzle.
I went ahead and went with the #67 jets and the #28 nozzle. This
combination works great! The Edelbrock Performer and the Carter AFB is
also an excellent choice. From what I have observed from owners of both
Holley and Edelbrock/Carter carbs it seems to be the general consensus
that AFB-based carbs are a little better overall than the Holleys.

There is no doubt about it -- more cubic inches equals
more hose power. Why do you think most people choose to swap in a 429/460?
It's because they are simply bigger, more powerful engines. However, you
can add more cubic inches (CID) to the 351M/400 (especially the
351M).

It is common practice when building an engine for more
power to increase an engine's size (measured in Cubic Inch Displacement --
CID for short) by enlarging the engines bore and/or stroke. The bore of an
engine is the diameter in inches of the piston (or cylinder bore) and the
stroke is the distance measured in inches in which the piston travels up
and down within the cylinder bore. The bore and stroke of the 400 is
4.0" x 4.0" and the bore and stroke of the 351M is 4.0" x
3.5". The formula for calculating engine displacement is:

This engine, by the way, it the Ford 302! This formula will
work with any engine. For example, the Oldsmobile 350 V8, which has
a bore of 4.057" and a stroke of 3.385" would be;

2.0285² × 3.14 × 3.385 x 8 = 349.88742333922
CID

A Ford 351 with a bore of 4" and a stroke of 3.5" would
be;

2² × 3.14 × 3.5 × 8 = 351.68
CID

Using this simple formula, you can calculate exactly
how much larger your engine will be when you enlarge the bore or lengthen
the stroke.

So, how can you increase your engine's CID? Well, if
you read the article "Dispelling the Myths & Untruths surrounding the
351/400", then you already know that you can not increase the bore
size of the 351M/400 more than 0.040 of an inch. If you plug this number
into the formula above on a 351M engine you will see that you can enlarge
it to 359 CID. (2.02² x 3.14 x 3.5 x 8 = 358.748738 CID). Also,
there are no aftermarket crankshaft kits that I know of that will lengthen
the stroke of the 351M/400. However, if you have a 351M then you are in
luck because you can install a 400 crank shaft in your engine to make your
engine a full 400 CID.

IMPORTANT:

When installing a 400 crankshaft in a 351M, there a
number of things that must be done and some things to keep in mind in
order to ensure a proper installation.

You MUST use 400 pistons with a 400
crankshaft -- Even though the 351M and the 400 have the size
diameter piston (4.0") the compression height of the 351M is much
higher than the 400. If 351 pistons are used with a 400 crankshaft
then the top of the pistons will slam into the valves and ruin the
whole engine the first time you turn it over.

The 351M uses a different harmonic balancer
than the 400 -- It is best to buy a new 400 harmonic balancer to
go along with a 400 crankshaft. You can use a 351M balancer,
however, you will have to rebalance the 400 crankshaft by adjusting
the counter weights to bring the assembly into balance. Also, the
timing marks on the 351M balancer will be nearly useless on a 400
crank -- I know because the engine shop used the 351M balancer on my
engine. I had to set my timing at 20+ degrees to get the engine
properly timed.

The 351M uses the SAME connecting rods as the
400 -- One advantage to this upgrade is the you will not have
buy new connecting rods because they are interchangeable between the
two engines. However, you may end up with an extra set of connecting
rods anyway because it is cheaper to buy a 400 crank kit as a
pre-balanced assembly. This is what I did and I came out ahead on
the cost.

Be prepared to pay a little extra for a shop
to perform this upgrade -- Engine shops like to use your
existing connecting rods and crank shaft when performing a rebuild.
It is easier for them to regrind you original crankshaft and install
the proper size bearings. The shop that built my engine charged me
an addition $100.00 to perform this upgrade for the extra trouble
they had to go through.

So how many more cubic inches can you squeeze out a
351M using a 400 crank? With a .030 over bore the engine's final size will
be 407.971408 CID! (or 408 CID).

If you stick with the guide lines I have given here in
this article, I promise you that you will have an engine that will
absolutely kick ass!!! In addition to the modifications I made to mine
that I've listed above, I also performed the following things to the
engine:

So how much did all this cost?
Everything, including all of the parts, labor, machining, and even the
intake and carburetor, cost right at $3,400.00 which is less than the
price of crate motor with much less performance upgrades than this engine
has! And to top it all off, my block and heads are original, so all the
numbers on this Bronco still match, which will go a long way in car show
should I ever choose to show it one
day.

How to Rebuild Ford Engines by Tom Monroe is by far the best and
most comprehensive book ever written in regards to 351M/400 engines.
The book covers nearly everything you ever wanted to know (and some things
you didn't) about the M-Block engines.